# CVE-2026-109391: Square Squash through commit e73f280 Unauthenticated YAML Deserialization via Symbolication API

> Unauthenticated attackers can achieve remote code execution in Square Squash by submitting malicious YAML payloads to the symbolication API.

- **Severity:** Critical 9.2 (`CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 12 Oct 2026
- **Updated:** 27 Oct 2026
- **Also known as:** GHSA-f7j3-hchq-qp35

## What is CVE-2026-109391?

CVE-2026-109391 is a critical unsafe deserialization vulnerability in Square Squash, published on 12 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Square Squash through commit e73f280 contains an insecure deserialization vulnerability that allows unauthenticated attackers to store crafted YAML via the POST /api/1.0/symbolication endpoint. When occurrences referencing the attacker-controlled symbolication UUID are symbolicated, the stored values are passed to YAML.load with SafeYAML in unsafe mode, enabling arbitrary Ruby object deserialization and potential code execution.

Weakness: CWE-502 (Deserialization of Untrusted Data)

## Which versions are affected by CVE-2026-109391?


## How do I fix CVE-2026-109391?

No fix has been published for CVE-2026-109391 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-109391 being exploited?

There is no known exploitation of CVE-2026-109391 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-109391 and fix it if it is affected.

What it is: CVE-2026-109391 is a critical unsafe deserialization vulnerability in Square Squash, published on 12 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-109391 in the wild and no public exploit code.

Affected product: Square Squash. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-109391/ (Markdown: https://insights.sbomify.com/CVE-2026-109391/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-109391
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-109391
- OSV: https://osv.dev/vulnerability/CVE-2026-109391

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-109391/
Summaries are generated and may contain mistakes; the official description above is the record.